An oil-burning power plant that Potomac Electric Power Company is building at Chalk Point, Md., should be converted to coal immediately, even though it is 95 percent complete, D.C. People's Counsel Brian Lederer said yesterday.

In his broadest attack yet on Pepco's plans for new facilities, the District's consumer advocate claimed Pepco's customers could save $60 million over the next decade if the company switched its newest power plant from oil to coal.

Lederer said Pepco's decision to build an oil-burning plant despite soaring petroleum prices and potential oil embargoes is evidence the District of Columbia Public Service Commission needs to restrict Pepco's construction budget.

The Public Service Commission for months has been holding hearings and taking testimony on the local electric company's construction plans. Yesterday Pepco and Lederer filed their final arguments in the case, which Lederer contends could affect electric rates for the next 20 years.

Pepco officials rejected virtually every criticism raised by Lederer, defending their construction plans and claiming Lederer has underestimated the cost of converting the oil plant to coal.

The Chalk Point plant is 95 percent complete and will be needed to supply Pepco's customers in the summer of 1982, when the company plans to shut down two aging oil plants at Buzzard Point and Benning Road in the District, said Pepco Vice President David Boyce.

Lederer asked the PSC to force Pepco to delay completion of the Chalk Point plant until 1984 and immediately switch it to coal. He estimates it would cost $300 million to convert the plant to coal, but Boyce contended the figure is $350 million to $360 million, so high it wipes out any savings in fuel costs for at least 10 years.

"The main point is that we need that plant in 1982. We've got to have it to serve our customers," Boyce responded. "Those plants at Benning and Buzzard Point are antiques; they're not reliable."

Boyce said Pepco is studying plans to burn either a mixture of oil and finely ground coal or a slurry of coal and water in the Chalk Point plant and that either could prove cheaper than building a whole new coal boiler.

He noted that Chalk Point is the last plant Pepco plans to build until 1993. Other planned power plants have been shelved because use of electricity by Pepco's customers is increasing only 1.4 percent a year.

Pepco's rates have increased only 15 percent since 1977, while the consumer price index has shown 39 percent inflation in that period. Boyce said.

Lederer urged the PSC to play a greater role in drafting Pepco's construction plans, to require Pepco to revise its method of forecasting future energy needs and to force Pepco to devote more effort to energy conservation.